Among color display devices used for image display on a computer or television screen, a plasma display panel device has recently been drawing attention, as a large and thin color display device having light weight.
A plasma display device performs additive color mixing of three primary colors (red, green, and blue) to provide full-color display. For the full-color display, a plasma display device has phosphor layers for emitting the respective three primary colors, i.e. red, green, and blue. In discharge cells of a plasma display device, discharge of a rare gas generates ultraviolet light having a wavelength up to 200 nm. The ultraviolet light excites phosphors of respective colors to generate visible light of respective colors.
Known as phosphors of the respective colors are (Y, Gd) BO3:Eu3+ and Y2O3:Eu3+ for red emission, (Ba, Sr, Mg) O.aAl2O3:Mn2+ and Zn2SiO4:Mn2+ for green emission, and BaMgAl10O17:Eu2+ for blue emission, for example.
Among these, for a blue phosphor called BAM that includes BaMgAl10O17 as its base material, Eu, i.e. its center of emission, must be activated divalent, in order to improve emission luminance. Thus, this phosphor is fabricated by firing in a reducing atmosphere (see “Phosphor Handbook”, Phosphor Research Society, Ohmsha, pp. 170, for example.) This is because, if the phosphor is fired in an oxidizing atmosphere, Eu is activated trivalent and Eu cannot substitutes for the bivalent Ba position in its host crystal. For this reason, Eu cannot be an active emission center, and this deteriorates emission luminance. Further, Eu does not accomplish its original purpose, and generates red emission peculiar to Eu3+.
For a red phosphor, europium-activated yttrium oxysulfide (Y2O2S:Eu3+), because Eu must be activated trivalent, the phosphor is fabricated by firing in an oxidizing atmosphere. Meanwhile, for a phosphor in which its host crystal is made of an oxide, it is considered that oxygen atoms are deprived from the host crystal in firing and thus oxygen vacancy is generated in the phosphor. Disclosed as a method of recovering such oxygen vacancy is firing the materials in an inert gas containing oxygen to activate Eu trivalent and provide Y2O2S:Eu3+ (see Japanese Patent Unexamined Publication No. 2000-290649).
However, in comparison with an oxide phosphor fabricated by firing in an oxidizing atmosphere, for an oxide phosphor fabricated by firing in a reducing atmosphere, the reducing atmosphere tends to deprive oxygen from the host crystal and oxygen vacancy in the host crystal increases. Further, when the oxide phosphor that must be fired in a reducing atmosphere is fired in an oxidizing atmosphere, keeping the number of valences inherent in the activator is difficult.
In other words, when a phosphor having much oxygen vacancy in its host crystal is subjected to radiation of high-energy ultraviolet light (having a wavelength of 147 nm) generated by a plasma display device and ion impact caused by discharge, the phosphor is likely to degrade with time. This is because, in the sites having oxygen vacancy, the bond between atoms is weak, and application of high-energy ultraviolet light and ion impact to the sites tends to disturb the crystal structure and make the sites amorphous. The amorphous sites mean deterioration of the host crystal. In a plasma display device, such deterioration leads to luminance degradation with time, color shift caused by chromaticity change, and image burn.
When an oxide phosphor that must be fired in a reducing atmosphere is fired in an oxygen atmosphere for the purpose of recovery of oxygen vacancy, for a BAM phosphor, for example, Eu is activated to Eu3+ and this causes considerable luminance degradation.
The present invention addresses these problems, and aims to provide a method of fabricating a phosphor, and a plasma display panel using the phosphor. With the fabricating method, even for a phosphor in which its emission center, Eu or Mn, must be activated bivalent, and its host crystal is made of an oxide, its oxygen vacancy can be recovered without deterioration of emission luminance.